Open both images in Photoshop (ie, as separate documents).
Click somewhere in the dessert image to make it the active document.
Type control-A (for a PC) to select the entire 2000x2000 image of the dessert.
Type control-C to copy what you just selected.
Type control-F6 to switch to the other document, the monitor.
Type control-V to paste the image of the dessert as a new layer on the top of the layer stack in the monitor document.
Since the material you added is larger than the size of the original document, keep hitting control-(minus sign) to keep zooming out till you see the entirety of the dessert image.
Click the move tool. Usually it's on the LHS of your screen, at or near the top of all the tool icons. It looks like two double headed arrows crossed at 90 degrees.
You should now see tiny gray squares at each of the corners and along each of the edges of the dessert image. These are the handles which allow you to re-size and re-shape at image.
Click and hold the handle in the upper left hand corner (U-LHC) of the dessert image. Move your mouse diagonally till the handle is positioned at the U-LHC of the window area of the monitor image. Release the mouse button.
Do the same for the handle in the lower LHC of the dessert image. If the two images are perfectly square and both have a 1:1 aspect ratio, the desert image should now be located quite closely to where it needs to be and very close to the correct size.
Tweak as needed.
If you can't get it exact for some reason (eg, the monitor screen is not exactly 1:1 vert-to-horiz, the edges or corners of the monitor are slightly curved, the photo of the monitor was taken at an angle, you don't like the added image to have sharp edges, etc.), post the two images, and we can go from there.
Tom M
PS - Sorry there are no pix in the above tutorial, but I'm not at my computer at the moment.
PPS - There are other ways to do what you asked, but the above is one of the most straightforward and the technique involved, the Move tool, is important for many, many other uses of Photoshop.